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Wednesday, October 29, 2008
Exhibit Hall
A fuel cell model can be very helpful for addressing design flaws and performance issues in relation to operating conditions, for reviewing failure modes and for indentifying degradation mechanisms. However, a model must first be verified experimentally before being used by fuel cell researchers. To provide modelers with a platform for modeling and sufficient experimental data to verify their model, a diagnostic modeling PEM fuel cell with an active area of 192cm2 (8cm*24cm) has been designed and fabricated. This diagnostic modeling cell has been developed to provide both average performance and local information for model verification. The cell hardware features easy assembly and easy expansion from a single cell to a short stack. Through a bladder regulated by compressed air, the stack pressure can be continuously changed.
The flow field design was carried out based on Darcy’s equation in order to fulfill two objectives: 1) the cell should be able to operate at ambient pressure (close to zero pressure drop) and 2) the flow speed should be at least 7m/s for better water management. For the cathode, a straight channel configuration was able to meet the two objectives easily and in addition, the straight channel design simplified CFD modeling. But a straight channel design did not fulfill the two objectives for the anode. Thus a two-path serpentine channel design was used for the anode flow field. Micro thermocouples were also integrated into the anode plate for temperature mapping and current mapping.
A series of diagnostic tests were carried out with the diagnostic modeling cell. First, the performance of the diagnostic modeling cell was measured and the effects of flow rate, temperature, backpressure, stoichiometric number and relative humidity on the cell performance were investigated. Pressure drop between the inlet and outlet was measured with and without a gas diffusion layer. A comparison of the measured results with calculations by both Darcy’s equation and CFD model was carried out. Voltage variation along the flow field channel from inlet to outlet was measured at different current densities. Large voltage variation was seen at high current densities.
Temperature mapping at different operating conditions was carried out as well. Altogether 18 micro thermocouples were embedded in the anode plate. The effects of temperature, flow rate, relative humidity, backpressure, and stoichiometric number on the temperature variation across the active area were investigated with a constant coolant circulation rate. Since a relatively high coolant circulation rate was used in these measurements, only a small variation of local temperature across the active area was observed. However, different operating conditions did show effects on the temperature distribution. With a low coolant circulation rate, the effects on local temperature variation would have been more pronounced.
The flow field design was carried out based on Darcy’s equation in order to fulfill two objectives: 1) the cell should be able to operate at ambient pressure (close to zero pressure drop) and 2) the flow speed should be at least 7m/s for better water management. For the cathode, a straight channel configuration was able to meet the two objectives easily and in addition, the straight channel design simplified CFD modeling. But a straight channel design did not fulfill the two objectives for the anode. Thus a two-path serpentine channel design was used for the anode flow field. Micro thermocouples were also integrated into the anode plate for temperature mapping and current mapping.
A series of diagnostic tests were carried out with the diagnostic modeling cell. First, the performance of the diagnostic modeling cell was measured and the effects of flow rate, temperature, backpressure, stoichiometric number and relative humidity on the cell performance were investigated. Pressure drop between the inlet and outlet was measured with and without a gas diffusion layer. A comparison of the measured results with calculations by both Darcy’s equation and CFD model was carried out. Voltage variation along the flow field channel from inlet to outlet was measured at different current densities. Large voltage variation was seen at high current densities.
Temperature mapping at different operating conditions was carried out as well. Altogether 18 micro thermocouples were embedded in the anode plate. The effects of temperature, flow rate, relative humidity, backpressure, and stoichiometric number on the temperature variation across the active area were investigated with a constant coolant circulation rate. Since a relatively high coolant circulation rate was used in these measurements, only a small variation of local temperature across the active area was observed. However, different operating conditions did show effects on the temperature distribution. With a low coolant circulation rate, the effects on local temperature variation would have been more pronounced.
Current density distribution data was obtained by transforming the temperature distribution measurement results into current density distribution through the published technique that was previously established in this lab. This technique is based on the relationship between the local current density and the heat generated from the reaction under adiabatic conditions. The results showed that the local current density increased along the flow field channel from the inlet to a location close to the outlet and then decreased continuously until to the outlet. This trend is exactly opposite to that of the local voltage change.